Thermal spraying is a coating method wherein powder or other feedstock material is fed into a stream of heated gas produced by a plasmatron or by the combustion of fuel gasses. The feedstock is entrapped by the hot gas stream from which it is transferred heat and momentum and it is further impacted onto a surface where it adheres and solidifies, forming a relatively thick thermally sprayed coating by the cladding of subsequent thin layers or lamellae.
It has been recognized for some time that, in the case of some thermal spray applications, injecting feedstock axially into a heated gas stream presents certain advantages over traditional methods wherein feedstock is fed into the stream in a direction generally described as radial injection, in other words in a direction more or less perpendicular to the direction of travel of the stream. Such advantages of the axial injection relate mainly to the potential to control better the linearity and the direction of feedstock particle trajectory. It would be therefore desirable to inject feedstock in a manner that induces an optimal particle trajectory in the axial direction.
Plasma torches with axial injection of feedstock can be classified in two major groups: a) with multiple cathodes, also known as the pluri-plasmatron or the multiple-jet type; b) with single cathode, also known as the single stream type.
Examples of multiple cathode plasma torches with axial injection are found in U.S. Pat. No. 3,140,380 of Jensen, U.S. Pat. No. 3,312,566 of Winzeler et al., U.S. Pat. No. 5,008,511 of Ross and U.S. Pat. No. 5,556,558 of Ross et al. They show a plurality of plasmatrons symmetrically arranged about the axis of the plasma spray torch and provide for nozzle means to converge the plurality of plasmas into a single plasma stream. Feeding means are also provided to inject feedstock materials along the axis of the single plasma stream. Although such plasma torches can produce satisfactory coatings, they involve complex torch configurations as well as the use of multiple power supplies for the multiple cathodes. The use of multiple cathodes and multiple arc chambers, which need to be replaced regularly, induce high operating costs for such plasma torches. A different approach to achieve axial injection employing multiple cathodes and a complex single arc chamber configuration is found in U.S. Pat. Nos. 5,225,652 and 5,332,885, both issued to Landes.
The single cathode type plasma torches with axial injection have certain advantages such as a less complex torch configuration, less operating costs and less manufacturing costs for the plasma system. It has been recognized for some time that the introduction of powder axially through a central hole in the cathode tip is not an efficient solution for axial injection. Such an approach is found in U.S. Pat. No. 5,225,652 of Landes. The powder interferes with the electric arc, readily resulting in malfunctioning of the torch. Other arrangements for the single cathode approach are found in U.S. Pat. No. 4,540,121 of Browning, U.S. Pat. No. 4,780,591 of Bernecki et al., U.S. Pat. No. 5,420.391 of Delcea and U.S. Pat. No. 5,837,959 of Muehlberger et al. For example, Muehlberger et al. teach an output plasma nozzle oriented at an acute angle with respect to torch axis. A powder feed tube axial with the output nozzle opens at or about the bent in the plasma path or alternatively penetrates into the plasma stream. Both alternatives proposed by Muehlberger induce a non-uniform interaction between the plasma stream and the powder due to bending of the stream and the introduction of an angled tube in the path of the stream. The plasma stream has a lower density and velocity along the wall of the far side bent, which affects the trajectory of the powder. Bemecki et al. teach a semi-splitting of the plasma stream by means of an arm which protrudes radially into the plasma stream and connects to a core member positioned axially within the plasma torch nozzle. This approach creates an asymmetrical plasma stream at the point of powder injection, with a portion of the plasma stream going undisturbed about the injector, while the rest of the stream s split by the arm before the injection point. It is clear that if an additional arm was provided symmetrically in Bemecki, a symmetrical splitting and uniform interaction between the plasma stream and the powder would be achieved. This improvement is found in patent '391 of Delcea, which teaches a single step symmetrical splitting of a single plasma stream and the axial injection through the core member. One of the disadvantages common to the designs found in patents '591 and '391 is related to the short length of the feedstock input passage running axially inside the core. When using reasonable carrier gas flows, the carrier gas and the powder are bent at 90.degree. and cannot be accelerated sufficiently along the short feedstock passage in order to be efficiently projected axially into the plasma stream without being affected by turbulence. If higher carrier gas flows are used to more efficiently push the powder axially, the injection of the carrier gas will cool the plasma to the detriment of torch efficiency. On the other hand, if the feedstock input passage is extended sufficiently, the elongated core becomes exposed excessively to the hot plasma, with deleterious effects on the core and on the thermal efficiency of the torch. Patent '121 of Browning discloses a single cathode plasma torch which splits the plasma stream in a first plurality of equal streams and then further splits each of the first plurality of streams in a second plurality of equal streams symmetrically arranged about the core. The second splitting occurs simultaneously with bending of the streams at 90 degrees. The bending of the streams allows for an extended core and an extended powder feed channel. This approach apparently could solve the problem of powder axial acceleration affecting the torches shown in Bernecki and Delcea. However, Browning's torch has a complicated and complex configuration, and the torch is highly inefficient due to multiple turbulent disruptions of the plasma stream induced by multiple cascade-splitting and bending of the stream.
With respect to combustion spray torches, in a majority of cases the powder is injected radially at the inlet of an elongated output nozzle. In one prior art, U.S. Pat. No. 4,416,421 of Browning, the powder is injected axially in a flame-spray apparatus extremely similar to the plasma torch described by same Browning in U.S. Pat. No. 4,540,121. Therefore, the feedstock injection method described by Browning in Patent '421 presents the same disadvantages as described above with reference to the Browning Patent '121.
In the case of all thermal spray torches, it is of notorious practice to attach an output spray nozzle in order to increase feedstock velocity and the transfer of heat to the feedstock. As a general rule, the longer the output nozzle the more velocity is transferred from the gas stream to the feedstock and therefore denser thermal spray coatings can be obtained. One of the main factors that limit the size of the output nozzle is the trajectory of the molten feedstock along the nozzle passage. If the injection of the feedstock is such that at least some feedstock deviates towards the internal wall of the nozzle, it will solidify and build up on the cold surface of the internal wall therefore resulting in malfunctioning of the spray process.
Accordingly, it would be desirable to provide a feedstock injector for attachment to a single stream spray torch, the injector providing for optimal interaction between the feedstock and the gas stream.